The present invention relates to a method and apparatus for improving human hearing.
A common hearing problem involves the tympanic membrane of the middle ear. Sound pressure in the air is translated into pressure in the cochlea fluids of the inner ear through the tympanic membrane and the ossicular chain (the malleus, incus, and stapes) of the middle ear. When the tympanic membrane vibrates, the ossicular chain pushes on the oval window membrane of the cochlea to move the fluids of the inner ear. Because the inner-ear fluid is denser than air, sounds in the air would not be very efficient in directly moving the fluid. The fluid has a higher characteristic impedance than the air. The impedance mismatch means that more pressure is required for a stimulus to be propagated in the cochlear fluids than in air.
When stimulated by high sound pressure levels, the tympanic membrane operates as a stretched membrane, and portions of the air pressures acting on it are transferred to the manubrium of the malleus. About two-thirds of the tympanic membrane total area (typically about 55 mm.sup.2 of the total 85 mm2 area) is stiffly connected to the manubrium and thus vibrates at high levels. The stapes makes contact with the oval window and fluids of the inner ear. The area of the stapes footplate is about 3.2 mm.sup.2. Since the area of the stapes footplate into the inner ear is considerably smaller than the effective area of the tympanic membrane, the difference in surface area acts to increase the pressure exerted on the cochlea. The pressure transformation from the tympanic membrane to the inner ear depends upon the frequency of the acoustic stimulus.
A common hearing problem is caused by the loosening of the tympanic membrane. This loosening can be caused by heredity, a puncture in the tympanic membrane, some surgical procedures, loud noises, or aging. This loosening may shift the resonant frequency of the middle ear to a lower frequency and/or decrease efficiency of the middle ear structures, both of which can cause a hearing loss in the range of 1 kHz to 6 kHz, which is a crucial one for the hearing of human voices.
To avoid this problem, hearing aids are typically used to amplify sounds in the range of 1 kHz to 6 kHz. Alternately, a passive filter can be used to filter out some of the sounds in the lower frequency range so that voices do not seem as muffled. A problem with these devices is that people generally do not like to use them for comfort and cosmetic reasons.
It is desired to have an improved method for improving the hearing in the range of from 1 kHz to 6 kHz.